A. Field of the Invention
The present invention relates to production of color dye-transfer separations, or "proofing bases", and more particularly to a method for producing proofing bases using spark-discharge recording techniques and constructions therefor.
B. Description of the Related Art
Graphic-arts practitioners use color proofing sheets (or simply "color proofs") to correct separation images prior to producing final separation plates, as well as to evaluate the color quality that will be obtained during the printing process. In typical printing processes, multicolor images cannot be printed directly using a single printing plate. Rather, composite color images are first decomposed into a set of constituent color components, or "separations", each of which serve as the basis for an individual plate. The colors into which the multicolor image is decomposed depends on the particular "color model" chosen by the practitioner. The most common color model is based on cyan, magenta, yellow and black constituents, and is referred to as the "CMYK" color model. If the separation is performed properly, subtractive combination of the individual separations produces the original composite image. A color proof represents, and permits the practitioner to view, the final image as it will appear when printed.
The most common method of producing a color proof involves preparing translucent representations of each separation on transparent supports. A coloring agent corresponding to each separation color is applied to an individual transparent substrate according to the distribution of that color in the final image. The complete set of color proofs corresponding to a multicolor image may be superposed on each other in registration, thereby revealing the final image, or individually analyzed with respect to the corresponding separation printing plate.
The translucent images are often prepared using photopolymerization and thermal transfer techniques. First, black-and-white separations corresponding to each color of the color model are produced using conventional color-analysis methods. These separations are interposed between individual "proofing bases", which consist of a support coated with photopolymerizable material, and a suitable source of radiation. The radiation polymerizes the coating, rendering it unsuitable for transfer. The surface of each exposed proofing base is then pressed into contact with a sheet of the final transparency material, and one or both elements heated to a temperature above the transfer temperature of the unexposed (and hence unpolymerized) portions of the proofing base. This causes the unexposed transfer material to be laminated onto the transparency sheet. The proofing bases and transparency sheets are then separated. If the transfer material is not colored, the imaged transparent substrates can be subjected to the action of various colorant toners, which adhere only to the transfer material and not the transparency sheet. The exposure, transfer and toning steps must be carried out for each translucent image. Although widely employed, this process is time-consuming and not amenable to digital control.
U.S. Pat. No. 3,780,214 describes a thermal-transfer process for producing color prints in which a laser is employed either (1) to heat portions of a thermosensitive recording layer so as to facilitate transfer of these portions to a suitable substrate, or (2) to destroy portions of a recording layer so that such portions are unavailable for transfer. Laser equipment tends to be costly and, for the thermal processes envisioned in this reference, potentially slow-acting. In addition, the thermoplastic characteristics of the recording layer must be compatible with the particular laser, which limits the choice of recording materials.
The present invention utilizes contacting or non-contacting spark-discharge recording apparatus to create color-transfer proofing bases. In contacting spark-discharge apparatus, an electric current is applied from a needle electrode to the electroconductive layer of a suitable recording medium, causing a spark discharge therebetween. The effect of the spark can be to destroy the electroconductive layer or alter its surface characteristics. See, e.g., U.S. Pat. No. 4,596,733 and references discussed therein. The non-contacting apparatus causes a similar spark discharge to take place between a recording stylus and an electroconductive layer, but physical contact between these components is unnecessary. This latter approach allows greater flexibility in the choice of recording media, and also avoids unwanted scratch marks which sometimes result from electrode contact. Non-contacting spark-discharge recording apparatus are described in copending application Ser. No. 07/234,475 (commonly owned with the present application). Both contacting and non-contact apparatus are amenable to computer control.